Enable @typescript-eslint/no-unnecessary-condition

Workarounds:

- `false as boolean` to avoid TypeScript saying something is `false`
  (never `true`) where it doesn’t know that mutation elsewhere can set
  it back to `true`.
- Ignore “unnecessary” `switch` on unions with only one variant – so
  far.
- Ignore places where I do weird gymnastics with untrusted objects
  (where the types are more constrained than reality).

client/client.ts

@@ -21,6 +21,7 @@ import {
 // Support Web Workers, where `window` does not exist.
 const window = globalThis as unknown as Window;
 
+// eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
 const IS_WEB_WORKER = window.window === undefined;
 
 // These used to be separate properties on `window`, like
@@ -159,6 +160,7 @@ const DEFAULT_ELM_WATCH: __ELM_WATCH = {
 
 let { __ELM_WATCH } = window;
 
+// eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
 if (typeof __ELM_WATCH !== "object" || __ELM_WATCH === null) {
   // Each property is added later below.
   __ELM_WATCH = {} as unknown as __ELM_WATCH;
@@ -168,6 +170,7 @@ if (typeof __ELM_WATCH !== "object" || __ELM_WATCH === null) {
 }
 
 for (const [key, value] of Object.entries(DEFAULT_ELM_WATCH)) {
+  // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
   if (__ELM_WATCH[key as keyof __ELM_WATCH] === undefined) {
     (__ELM_WATCH as Record<string, unknown>)[key] = value;
   }

client/proxy.ts

@@ -19,6 +19,7 @@ error.elmWatchProxy = true;
 
 const existing = window.Elm;
 const existingObject =
+  // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
   typeof existing === "object" && existing !== null ? existing : undefined;
 
 const elmProxy = new Proxy(existingObject ?? {}, {

eslint.config.js

@@ -79,6 +79,7 @@ export default typescriptEslint.config(
       "@typescript-eslint/no-shadow": error,
       "@typescript-eslint/no-this-alias": warn,
       "@typescript-eslint/no-unnecessary-boolean-literal-compare": warn,
+      "@typescript-eslint/no-unnecessary-condition": error,
       "@typescript-eslint/no-unnecessary-parameter-property-assignment": warn,
       "@typescript-eslint/no-unnecessary-template-expression": error,
       "@typescript-eslint/no-unnecessary-type-arguments": warn,

src/Compile.ts

@@ -116,7 +116,7 @@ export function installDependencies(
       );
 
       // Avoid printing `loadingMessage` if there’s nothing to download.
-      let didWriteLoadingMessage = false;
+      let didWriteLoadingMessage = false as boolean;
       const timeoutId = setTimeout(() => {
         logger.write(loadingMessage);
         didWriteLoadingMessage = true;
@@ -691,7 +691,7 @@ async function compileOneOutput({
 
   // Watcher events that happen while waiting for `elm make` and
   // postprocessing can flip `dirty` back to `true`.
-  outputState.dirty = false;
+  outputState.dirty = false as boolean;
 
   const { promise, kill } = SpawnElm.make({
     elmJsonPath,
@@ -1630,8 +1630,7 @@ function targetNameEmojiTweak(
     return { targetName, delta: 0 };
   }
 
-  /* v8 ignore next */
-  const content = match[0] ?? "";
+  const content = match[0];
 
   // Avoid emoji on Windows, for example.
   if (!loggerConfig.fancy) {

src/Errors.ts

@@ -1746,6 +1746,7 @@ ${text(errorFilePath.content)}
 
 function printUnknownValueAsString(value: UnknownValueAsString): Piece {
   switch (value.tag) {
+    // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
     case "UnknownValueAsString":
       return text(value.value);
   }

src/Hot.ts

@@ -1459,6 +1459,7 @@ const runCmd =
           // Retry writing it.
           writeElmWatchStuffJson(mutable);
           // If still an error, print it.
+          // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
           if (mutable.elmWatchStuffJsonWriteError !== undefined) {
             logger.write("");
             logger.errorTemplate(

src/Postprocess.ts

@@ -255,6 +255,7 @@ class PostprocessWorker {
 
     this.worker.on("message", (message: MessageFromWorker) => {
       switch (message.tag) {
+        // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
         case "PostprocessDone":
           switch (this.status.tag) {
             /* v8 ignore start */

src/PostprocessWorker.ts

@@ -39,6 +39,7 @@ function main(port: PortWrapper): void {
 
   port.on("message", (message: MessageToWorker) => {
     switch (message.tag) {
+      // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
       case "StartPostprocess":
         elmWatchNode(message.args)
           .then((result) => {

src/index.ts

@@ -93,6 +93,7 @@ export async function elmWatchCli(
             );
           } while (result.tag === "Restart");
           switch (result.tag) {
+            // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
             case "Exit":
               return result.exitCode;
           }
@@ -126,6 +127,7 @@ if (require.main === module) {
       // happens it should at least be possible to exit with a simple ctrl+c.
       // Note: `.setRawMode` is `undefined` when stdin is not a TTY, but this is
       // not reflected in the type definitions.
+      // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
       if (process.stdin.setRawMode !== undefined) {
         process.stdin.setRawMode(false);
       }

tests/Errors.test.ts

@@ -1713,6 +1713,7 @@ describe("errors", () => {
           encoding: "utf8",
         },
       );
+      // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
       if (!(result.error === undefined || result.error === null)) {
         throw result.error;
       }

tests/HotHelpers.ts

@@ -39,6 +39,7 @@ const hotKillManager: HotKillManager = { kill: undefined };
 export async function cleanupAfterEachTest(): Promise<void> {
   const { currentTestName } = expect.getState();
 
+  // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
   if (window.__ELM_WATCH?.KILL_MATCHING !== undefined) {
     // The idea is that we need no logging here – it’ll just result in double
     // logging since there will most likely be a running server as well.
@@ -528,7 +529,7 @@ export function collapseUi(targetName?: string): void {
 
 function expandUiHelper(wantExpanded: boolean, targetName?: string): void {
   withShadowRoot((shadowRoot) => {
-    const button = shadowRoot?.querySelector(
+    const button = shadowRoot.querySelector(
       `${
         targetName === undefined
           ? "[data-target]"
@@ -547,7 +548,7 @@ function expandUiHelper(wantExpanded: boolean, targetName?: string): void {
 
 export function showErrors(targetName?: string): void {
   withShadowRoot((shadowRoot) => {
-    const button = shadowRoot?.querySelector(
+    const button = shadowRoot.querySelector(
       `${
         targetName === undefined
           ? "[data-target]"
@@ -564,7 +565,7 @@ export function showErrors(targetName?: string): void {
 
 export function hideErrors(targetName?: string): void {
   withShadowRoot((shadowRoot) => {
-    const button = shadowRoot?.querySelector(
+    const button = shadowRoot.querySelector(
       `${
         targetName === undefined
           ? "[data-target]"
@@ -581,7 +582,7 @@ export function hideErrors(targetName?: string): void {
 
 export function closeOverlay(): void {
   withShadowRoot((shadowRoot) => {
-    const button = shadowRoot?.querySelector(
+    const button = shadowRoot.querySelector(
       `[data-test-id="OverlayCloseButton"]`,
     );
     if (button instanceof HTMLElement) {
@@ -595,7 +596,7 @@ export function closeOverlay(): void {
 export function getOverlay(): string {
   let result = "(Overlay not found)";
   withShadowRoot((shadowRoot) => {
-    const overlay = shadowRoot?.querySelector(`[data-test-id="Overlay"]`);
+    const overlay = shadowRoot.querySelector(`[data-test-id="Overlay"]`);
     if (overlay instanceof HTMLElement) {
       const children = Array.from(overlay.children, (child, index) => {
         const clone = child.cloneNode(true) as HTMLElement;
@@ -617,7 +618,7 @@ export function getOverlay(): string {
 
 export function clickFirstErrorLocation(): void {
   withShadowRoot((shadowRoot) => {
-    const button = shadowRoot?.querySelector(`[data-test-id="Overlay"] button`);
+    const button = shadowRoot.querySelector(`[data-test-id="Overlay"] button`);
     if (button instanceof HTMLButtonElement) {
       button.click();
     } else {
@@ -629,7 +630,7 @@ export function clickFirstErrorLocation(): void {
 export function moveUi(position: BrowserUiPosition): void {
   expandUi();
   withShadowRoot((shadowRoot) => {
-    const button = shadowRoot?.querySelector(
+    const button = shadowRoot.querySelector(
       `button[data-position="${position}"]`,
     );
     if (button instanceof HTMLButtonElement) {
@@ -643,7 +644,7 @@ export function moveUi(position: BrowserUiPosition): void {
 export function switchCompilationMode(compilationMode: CompilationMode): void {
   expandUi();
   withShadowRoot((shadowRoot) => {
-    const radio = shadowRoot?.querySelector(
+    const radio = shadowRoot.querySelector(
       `input[type="radio"][value="${compilationMode}"]`,
     );
     if (radio instanceof HTMLInputElement) {
@@ -657,7 +658,7 @@ export function switchCompilationMode(compilationMode: CompilationMode): void {
 export function assertCompilationMode(compilationMode: CompilationMode): void {
   expandUi();
   withShadowRoot((shadowRoot) => {
-    const radio = shadowRoot?.querySelector(`input[type="radio"]:checked`);
+    const radio = shadowRoot.querySelector(`input[type="radio"]:checked`);
     if (radio instanceof HTMLInputElement) {
       expect(radio.value).toStrictEqual(compilationMode);
     } else {
@@ -671,7 +672,7 @@ export function assertCompilationMode(compilationMode: CompilationMode): void {
 export function assertDebugDisabled(): void {
   expandUi();
   withShadowRoot((shadowRoot) => {
-    const radio = shadowRoot?.querySelector('input[type="radio"]');
+    const radio = shadowRoot.querySelector('input[type="radio"]');
     if (radio instanceof HTMLInputElement) {
       expect(radio.disabled).toBe(true);
     } else {